Quantifying Shade Produced by Desert-Adapted Landscape Trees
Landscape architects frequently ask whether desert-adapted landscape trees generate as much shade as non-desert tree species. In 2018, Arid Zone Trees, in cooperation with Enumeral Research and Consulting, LLC, undertook a research project to attempt to quantify soil surface temperature reductions provided from the shade canopy of desert-adapted landscape trees.
The ALCA Influence March/ April issue includes an article summarizing preliminary results of this study conducted by Ed Mulrean, Ph.D., Justin Morrow and Kevin Salamandra. Both container grown and landscape planted trees were included in the study. The article concludes, “While additional work is needed to fully assess desert trees’ potential to offset the urban heat island, these results suggest that reductions of at least 14 to 30% (20 to 43° F) are possible. “This study will continue through 2019 and possible longer. To read the full article and review the data presented see, “Quantifying Shade Produced by Desert-Adapted Landscape Trees.”
Cercidium (Parkinsonia) hybrid ‘AZT™’
Nothing heralds the arrival of the desert spring (and in some year’s the start of 100-degree temperatures) as brilliantly and conspicuously as the blooming of the Palo Verdes. With a succession that includes Blue (Cercidium floridum), Foothill, Mexican and Sonoran Palo Verdes color the desert a brilliant yellow from spring to early summer. Other than Mexican Palo Verde (Parkinsonia aculeata) all these Palo Verdes are popular and widely used landscape trees. The various Palo Verde species are so closely related, botanically and genetically, that hybridization between species regularly occurs in both native and nursery grown populations. In the past this hybridization went largely unnoticed, ignored or was mistaken for acceptable genetic variation within a single species. The potential for Cercidium hybrids to offer genetic advantages like increased vigor, greater cold hardiness, enhanced flowering and the absence of thorns, brought them to the attention of horticultural professionals and propagators.
Cercidium (Parkinsonia) hybrid ‘AZT™’ exhibits the form, structure and grace found in Palo Brea, Blue and Mexican Palo Verde in combination with a leaf canopy that provides ample shade and lush, green leaves. These trees are thornless and produce brilliant yellow flowers in abundance in spring, intermittently during the summer months and occasionally in the fall. Similar to the Palo Brea, the branches and trunks of variety 'AZT™' remain a smooth, lime green as they mature and possess the genetic capacity to be grown as either V-shaped upright or candelabra form specimens. The orientation and angles of branches are also reminiscent of Palo Brea and make them ideal as “focal point” single specimens, theme trees, street and perimeter plantings and can be dramatically up-lit at night. Cercidium hybrid ‘AZT™’ has 4 to 6 pairs of leaves with 15 to 24 pairs of leaflets per leave. Their highly divided leaves produce a canopy that is lush yet airy and provides welcome filtered shade that promotes flowering of under-story shrubs and groundcovers. This hybrid is thornless, and semi-evergreen.
Variety 'AZT':
More than a decade ago Arid Zone Trees (AZT™) began vegetative propagation (cloning) of some of our most popular desert adapted tree species. This process involved: 1) identifying trees that had both desirable physical qualities (branching habits, leaf color, leaf canopy, and flower color) and sound horticultural characteristics (rooting, cold hardiness and growth rate); 2) developing propagation techniques that allowed the large-scale vegetative propagation of the selected trees; and 3) growing these selections to market sizes for additional nursery evaluation and field testing in landscape setting. This tree is the product of our research and evaluation. We have identified each of our clone species with the variety designation ‘AZT™’. Our rigorous screening and propagation process, in combination with AZT™’s Root Management Program, insures that Cercidium hybrid ‘AZT™’ brings beauty, durability and sound genetics to desert landscapes.
Cultural Practices:
Wind damage and uprooting are the products of poor pruning, staking practices, and improper irrigation and should not be considered as inherent, unavoidable horticultural shortcomings of Cercidium (Parkinsonia) species. Foster the development of a more dispersed root system and reduce the risk of wind throw by arranging irrigation emitters at varying distances from the trunk to encourage roots to seek out water and nutrients. Frequent watering is needed to promote good terminal growth on newly transplanted trees. Irrigation emitter arrangement along with other information about planting and irrigation practices for desert trees can be found at Irrigation Practices for Desert Trees and Tree Planting Practices an Overview.
Prune as needed to reinforce the structure and form of the tree. Periodic thinning is the most desirable method of pruning. Pruning to remove about 20% of the canopy during the growing season helps promote root development that is proportional to the shoot growth of young trees. Removing more than 20% of the canopy can inhibit rooting, lead to sunburn injuries that can later be invaded by wood boring insects, and encourage undesired re-growth made up of dense flushes of branches and leaves. Selective pruning should be used to promote the development of a symmetrical canopy with well-spaced branches. Avoid hedging or heading back desert species, as this will only stimulate excessive branching. Use tree stakes only when absolutely necessary and then only briefly. Select low-breaking, upright trees as they occupy no more space than a single-trunk specimen yet retain the natural wind resistance of trees found growing native in desert settings. Always use clean, sharp tools that are cleaned regularly in a 10% solution of bleach. For detail pruning guide see Pruning Desert Trees.
Periodically, insect pests can be a problem on some desert trees. On young trees, insect infestation can slow typical seasonal growth. Inspect trees during the growing season for common garden sucking insects such as aphids, thrips, whiteflies or psyllids. During dry months, (May and June) in dusty conditions, spider mites can appear. Monitor for infestation and apply controls as needed. Spray applications of water or water and Safer Soap give short-term control (3 to 7 days) for small insect population. For heavy infestation or longer control use federally registered insecticides. A contact insecticide application will kill existing adults. An application with a systemic soil drench will provide 8 to 12 weeks control for any post application insect hatchings or migration of insects. Before using pesticide for the first time or on new plants or cultivar, treat a few plants and check for phytotoxicty. Always read label and follow label instruction before using pesticides. For pesticide control recommendations contact a licensed pest control advisor.
It's Never Too Early to Think About a Spring Tour at AZT.
Have new employees? Need to take a fresh look at trees you haven't specified in a while? Want to get out of the office for an educational and refreshing field trip for you and your staff? It's never too early to be thinking about scheduling a tour of AZT's gardens and inventory in the spring of 2019. Contact Ed at ed@aridzonetrees.com, Kevin at kevin@aridzonetrees.com or call 480-987-9094 if you have questions or want to start looking at tour dates for your office.
UPDATE: The demonstration garden at AZT was always open to design professionals but that location will no longer be available for tours. Aside from enjoying a desert spring day in lovely surroundings, the purpose of the event was to encourage the ASLA design community that the Douglas Garden is available for you to tour with clients and colleagues. If you would like to arrange a tour of the Douglas Garden contact Kevin Salamandra (Kevin@aridzonetrees.com), Ed Mulrean (mulreaned@gmail.com) or info@aridzonetrees.com.